To achieve high-capacity fiber optic communications, wavelength division multiplexing (WDM) technology is being adopted. In WDM, a tunable light source (TLS) capable of emission at different wavelengths is used. A wavelength monitor circuit is provided in a tunable light source to control the wavelength. In general, a wavelength monitor circuit is implemented by combination of a wavelength or spectral filter having a periodic transmission spectrum and a photodiode (PD). A delay interferometer is used as such a wavelength filter. See, for example, U.S. patent application publication No. 2015/0085292 A1 and International Publication No. WO 2016/010528.
FIG. 1 illustrates a configuration of a conventional tunable light source. By having a light beam output from a semiconductor optical amplifier (indicated as “SOA1”) bounce between a resonator filter and the SOA1, light with a specific wavelength is amplified selectively. A portion of the amplified light is taken out from tap-1 and branched into two at tap-2. One of the branched light components is input to a wavelength filter, and the other is amplified by and output from another semiconductor optical amplifier (indicated as “SOA2”). A portion of the output light is received by a power monitoring photodiode “PDm”. The wavelength filter has a 3 dB coupler, a 90-degree hybrid coupler, and two waveguides extending at different lengths between the 3 dB coupler and the 90-degree hybrid coupler. Two light components output from the 90-degree hybrid coupler, with optical phases offset by 90 degrees from each other, are received at PD1 and PD2, respectively.
Photocurrents detected by the PD1 and the PD2 of the wavelength monitor circuit are denoted as P1 and P2, respectively, and photocurrent detected by the power monitoring photodiode “PDm” is denoted as Pmon. FIG. 2 illustrates spectra of two current ratios (i.e., transmission spectra of the wavelength filter) as a function of wavelength. The current ratio of P1 to Pmon (P1/Pmon) is indicated by a solid curve and the current ratio of P2 to Pmon (P2/Pmon) is indicated by a dashed curve. In order to precisely control the wavelength so as to be consistent with the respective wavelength gridlines, it is preferable for the filtering spectrum to have a greater slope. The greater the slope, the more sensitive the wavelength control with more derivative gain. At each wavelength gridline, whichever of the P1 and the P2 with a greater slope is selected, and the wavelength is controlled so as to bring the photocurrent of PD1 or PD2 to be the target value.
With the configuration of FIG. 1, the wavelength monitor circuit cannot achieve satisfactory accuracy because the intensity of light received at the power monitor “PDm” fluctuates depending on the state of the semiconductor optical amplifier “SOA2”. Another known structure of wavelength monitor circuit is illustrated in FIG. 3. A portion of light guided to the wavelength monitor circuit is branched at tap-3, and received by an additional photodiode PDmW for wavelength monitoring. The light received at PDmW is used as reference light.
In the configuration of FIG. 3, the additional tap-3 is provided and the quantity of light received at PD1 and PD2 is reduced. If the split ratio (or coupling ratio) of tap-2 is adjusted so as to allocate adequate light to the wavelength monitoring site, the intensity of light output from the SOA2 will decrease. Besides, the coupling ratio of tap-3 depends on the wavelength. In order for acquiring a quantity of light needed for the wavelength monitoring photodiode PDmW, the coupling ratio of tap-3 is determined on the basis of the wavelength resulting in the smallest proportion of light supplied to PDmW. This is because, for the wavelength resulting in the largest proportion of light supplied to the PDmW, the quantity of light received at PD1 and PD2 decreases, and therefore, the coupling ratio at tap-2 toward the wavelength filter needs to be increased. In this case, the power of light output from the light source decreases due to the increased optical loss. In addition, because the coupling ratio of tap-3 depends on the wavelength, another problem arises such that the controlled wavelength itself may shift and that the wavelength filter becomes unstable.
There is a demand for a wavelength tunable light source using a wavelength monitor with stable operation and less optical loss.